1. Field of the Invention
The present invention concerns viscometers. It is directed particularly to the type that drives a bob in alternating directions through the liquid to be measured and infers the liquid""s viscosity from the duration of a bob stroke.
2. Background Information
U.S. Pat. No. 4,864,849 to Hubert A. Wright, which is hereby incorporated by reference, describes a type of viscometer that is particularly simple mechanically. A bob containing ferromagnetic material is disposed in a channel that a liquid to be measured can enter. A coil is so positioned that the magnetic field caused when current flows through it tends to draw the bob in one direction along the channel. A second coil is so positioned as to draw the bob along the channel in the opposite direction. Driving first one coil and then the other applies an alternating magnetic force to the ferromagnetic-material-including bob, and the viscosity of the liquid through which the bob is thus driven can be inferred from a speed at which it travels through the liquid in response to these magnetic forces.
This use of coils to drive the bob is advantageous because the same coils can also be used for the bob-position sensing that inferring viscosity from bob speed requires. The Wright patent mentioned above describes a convenient approach to using the coils for such sensing. A small AC signal is superimposed on the DC level used to drive the coil that attracts the bob, and the magnetic-field component resulting from the driven coil""s AC current causes an AC voltage in the non-driven coil. The non-driven coil is coupled to a filter, which, among other things, increases the system""s signal-to-noise ratio. Because the bob includes ferromagnetic material, coil inductance varies with bob position. In the Wright arrangement, the variation is such that the resultant filter-output amplitude increases to a maximum when the position of the bob""s ferromagnetic material is approximately symmetrical with respect to the coils, and the amplitude decreases thereafter. The Wright arrangement concludes that the bob has reached the end of its travel when that output""s magnitude falls to some predetermined percentage of the maximum that it had attained during the bob stroke. The current drive is then switched from one coil to the other, and the liquid""s viscosity is inferred from the time that elapses between end-of-travel detections.
The approach that the Wright patent describes is quite effective, but it has to include provisions that compensate for the effects of delays that result from the need to enhance the system""s signal-to-noise ratio by filtering the non-driven coil""s output. In a given installation, the viscometer may be intended for use in measuring the viscosity of a relatively viscous liquid, but that liquid""s flow through a conduit that the viscometer monitors may be interrupted from time to time by flow of very-low-viscosity liquid. An example occurs in printing-industry installations when an ink-color change takes place and a low-viscosity solvent is used to flush the previous ink color out of the ink lines. The bob travel through the low-viscosity solvent can be too fast that for the detector""s filter to follow variations in the non-driven coil""s output with any precision. As a result, the filter output does not vary enough to meet the criterion that the system employs to recognize the bob""s having reached its predetermined end-of-travel position. The system would therefore fail to switch coil drive in the absence of some contrary provision.
Systems that have employed the Wright approach have therefore included provisions for switching coil drive if the system fails to detect the end-of-travel position within a timeout period whose duration exceeds a stroke duration corresponding to the highest expected viscosity. But suppose that the stroke duration corresponding to the highest viscosity intended to be measured is a full minute. That means that system flushing with a very-low-viscosity solvent would cause a delay of at least a minute before the viscosity of a subsequent, higher-viscosity liquid can be measured.
To reduce this delay, some users have made the timeout-interval duration adjustable, setting it to the sum of some safety margin and the most-recent valid stroke-duration measurement. When the unit times out, they gradually increase the timeout duration until there is a valid end-of-stroke detection before the timeout period ends, presumably because the next, higher-viscosity fluid has begun to flow. When the viscosity of the previous liquid is significantly less than the high end of the intended viscosity range, the shortened timeout period results in less delay.
We have developed a way reducing the delay even further. In accordance with our invention, the timeout-interval duration that prevails after a timeout has occurred is kept constant through subsequent cycles until a predetermined time period has elapsed, at least if no valid detection occurs in the interim. When that timeout period ends, the timeout-interval duration will typically be increased immediately to a high value.
We have recognized that such an approach has the potential to make the viscometer respond more quickly to transients of the type mentioned above. Timeouts usually are the result of the viscometer""s encountering a solvent or some other low-viscosity liquid, as was mentioned above, and, in most environments, the approximate duration of the solvent""s flow is known ahead of time. The predetermined time period for which the timeout-interval duration is kept constant will usually be chosen to approximate the expected time of solvent flow, so a valid measurement can usually be based on the first stroke after the timeout interval is raised again. And, if the constant timeout-interval duration is relatively low, the resultant rapid bob reciprocation fills the viscometer""s bob chamber more rapidly with the next, higher-viscosity liquid that the viscometer can measure.